This invention relates to the acoustic imaging of objects using a liquid crystal cell, and more particularly, to an improved sound source for use in such imaging.
The use of ultrasonics to nondestructively and noninvasively inspect objects for internal discontinuities, internal features, irregular shapes, etc., is known in the industrial fields and medical fields.
In one form, ultrasonic imaging is accomplished using electronic displays in which the ultrasonic signal is electronically detected (for example by a piezoelectric detector) and the image is electronically enhanced.
In another form, the ultrasonic image is detected and displayed using a liquid crystal cell or display which includes a layer of liquid crystal material encapsulated between a pair of cover plates. See, for example, Greguss, U.S. Pat. No. 3,841,434 and Dion, U.S. Pat. No. 4,338,821. Improvements in such cells are disclosed in Sandhu, U.S. Pat. No. 4,379,408.
Liquid crystal systems may be preferred as they are less complex and they do not require complicated signal processing in order to produce an image.
These ultrasonic liquid crystal systems include a sound source which generates an ultrasonic beam or acoustic field, which is directed at an object and insonifies the object. The liquid crystal cell detects the acoustic energy which passes through the object or is reflected from internal features of the object and displays an image.
The insonifying acoustic field may not be uniform, and as a result, the image may exhibit undesirable artifacts due to intensity variations in the plane of the cell. These intensity variation artifacts are believed to be due to constructive and destructive interference of the waves in the ultrasonic beam, which are often referred to as Fresnel or Fraunhofer patterns.
Commercial ultrasonic transducers include a piezoelectric plate, usually the face plate, which is electrically excited for producing the acoustic energy or an ultrasonic beam. These transducers direct the generated energy along a path generally normal to the face plate toward the object to be insonified. Energy exciting or reflected from the object is detected by and displayed on a liquid crystal cell. If the beam is viewed in cross section and at different distances from the transducer or face plate with a liquid crystal cell, the beam exhibits areas of high and low intensity. In the near field these are sometimes referred to as Fresnel patterns. The patterns differ with distance from the transducer face with piezoelectric element geometry and with the electrode pattern on the piezoelectric element. These patterns are believed to be formed by the constructive and destructive interference of acoustic waves from the transducer. See Physical Principles of Ultrasonic Diagnosis, by P. N. T. Wells, Academic Press 1969 at page 54 for a diagram showing such patterns, the distance between patterns and the correspondence of the intensity maximas and minimas along the central axis. These intensity variations appear as artifacts which are superimposed on, or imposed over, the image of the cell, thereby degrading the image quality.
Prior sound sources or transducers include: stationary transducers; sequentially activated or excited transducer arrays (e.g., phased arrays or linear arrays, etc.); transducers which are movable (e.g. by rotation) in a plane normal to the beam path; and transducers in which frequency and phase are controlled to render the beam and image more uniform. These prior sound sources are suitable for use in particular systems, such as pulse echo imaging, but may not be as suitable or useful in systems where highly attenuative objects are to be imaged using liquid crystals.
It is therefore an object of this invention to provide a system for producing a strong and uniform acoustic field for the insonification of objects and for use in displaying images on a liquid crystal cell.
It is a further object to eliminate or minimize the effect of constructive and destructive interference patterns caused by the sound source.
It is yet another object to minimize undesirable superpositions appearing in a liquid crystal detector cell.
Still another object is to provide a sound source particularly for use in imaging highly attenuative objects with liquid crystals.
These and other objects of this invention will become apparent from the following disclosure and appended claims.